1. Field of the Invention
This invention generally relates to image processing technology, and, in particular, to a system for converting a multilevel digital data into binary data with the use of thresholding technique. More specifically, the present invention relates to a system for converting a multilevel digital or analog video signal into a binary video signal suitable for use in image processing machines such as facsimile machines, digital copiers and optical readers.
2. Description of the Prior Art
FIG. 1 shows a typical prior art system for converting an analog video signal into a binary video signal. As shown, analog video signal (a) obtained by scanning an original with a photoelectric image sensor such as a CCD scanner is input to a peak detector 1 which then supplies as its output a peak level voltage signal (b). Then, the peak level voltage signal (b) is divided into approximately half by means of a voltage divider 2, and such a divided voltage signal (c) is then supplied to a comparator 3. Using the divided voltage signal (c) as a threshold for conversion to binary data, the comparator 3 processes the analog signal (a) thereby providing a binary converted video signal (d) as its output. FIG. 2 is a time chart showing the relation between the various signals indicated in FIG. 1. As is obvious from this time chart, in accordance with the above-described prior art system, since the threshold level is determined in an analog manner, it is extremely difficult to determine the threshold value by taking into account the density average of one or more of the previous scanning lines. As a result, some delicate information tends to be lost when converted into binary video data. For example, as best shown in FIG. 2, a collection of thin lines A would be converted into a single fat line and pale or faint lines B and C would disappear completely.